1. Field of the Invention
The present invention relates to a high frequency heating apparatus such as a microwave oven, and more specifically to a high frequency heating apparatus for heating an object to be heated inside a heating chamber by a magnetron or the like.
2. Description of the Background Art
A microwave oven is a typical example of a high frequency heating apparatus.
FIG. 12 shows a first prior art example. A microwave oven 100 shown in FIG. 12 is provided with a magnetron 101, a wave guide 102, and a heating chamber 103. Magnetron 101 is provided with a magnetron antenna 101A. Magnetron antenna 101A projects into wave guide 102. Wave guide 102 connects magnetron 101 and heating chamber 103. The microwaves generated in magnetron 101 are introduced into heating chamber 103 via wave guide 102.
Wave guide 102 is conventionally formed as a casing whose dimension does not change in the horizontal direction. As a result, it is difficult to match the impedances of magnetron 101 and heating chamber 103. Consequently, a high ratio of the microwaves generated in the magnetron is reflected toward the magnetron in a microwave oven as the one shown in FIG. 12, which leads to the problem of the heating efficiency of magnetron being poor.
FIG. 13 shows a second prior art example. A microwave oven 200 shown in FIG. 13 is provided with a magnetron 201, a wave guide 202, and a heating chamber 203. Magnetron 201 is provided with a magnetron antenna 201A. Magnetron antenna 201A projects into wave guide 202. In addition, wave guide 202 is provided with a metal plate 210 for controlling the direction of radiation of the microwaves generated in magnetron 201 to a desired direction. Metal plate 210 is rotatable by a movable member such as a motor, not shown.
A lower light surface of wave guide 202 has a slope. Thus, the area of the vertical cross section of wave guide 202 gradually increases in the direction of the microwave propagation, i. e. from magnetron 201 toward heating chamber 203. As a result, the impedances between a magnetron and a heating chamber can be matched more easily in the microwave oven shown in FIG. 13 than in the microwave oven shown in FIG. 12, and the heating efficiency can be improved in the microwave oven shown in FIG. 13.
In the microwave oven shown in FIG. 13, however, the cross-sectional area in the above-mentioned direction changes substantially throughout the entire wave guide 202. The positional relation between magnetron 201 and wave guide 202 would thus greatly affect the above-described impedance matching. In other words, an error in mounting position of magnetron 201 would affect the heating capabilities of the microwave oven, which leads to the problem of unstable heating efficiency in the microwave oven shown in FIG. 13.
Moreover, metal plate 210 made rotatable by a movable member is provided in the microwave oven shown in FIG. 13. It may be preferable to match the impedances of the magnetron and the heating chamber by providing a movable member like metal plate 210 and by moving this movable member in an appropriate manner.
A movable member in general, however, has by comparison a more complicated structure than a non-movable member that is simply mounted so that the possibility of malfunctioning is greater in the former. Therefore, the provision of a movable member for the purpose of impedance matching in a microwave oven may, instead, create a new problem of unstable heating capabilities of the microwave oven due to the malfunction of the member.
On the other hand, some conventional microwave ovens provide the microwaves into the heating chamber from one side surface of the heating chamber as shown in FIGS. 12 and 13, while other conventional microwave ovens provide the microwaves into the heating chamber from the top surface and the bottom surface of the heating chamber. FIG. 14 shows a third prior art example.
A microwave oven 300 is provided with a heating chamber 303, magnetrons 301 and 304 for heating an object to be heated 316 inside heating chamber 303 by generating the microwaves, and wave guides 302 and 305 for introducing into heating chamber 303 the microwaves generated by magnetrons 301 and 304, respectively. Radiation apertures 313 and 319 are respectively provided on the top and the bottom of heating chamber 303, and the microwaves guided through wave guides 302 and 305 are provided to heating chamber 303 via the respective radiation apertures 313 and 309. In addition, a turntable on which the object to be heated is to be placed is denoted by 314, and a turntable motor for rotating turntable 314 is denoted by 315 in the drawing.
Turntable 314 is preferably made of a microwave-permeable material (such as glass) alone. It, however, is normally difficult to form the turntable only of a material such as glass due to considerations of mechanical strength and mechanical connection to be established with turntable motor 315. Thus, turntable 314 in a conventional microwave oven is formed by a combination of a metallic receiving base and a plate made of glass or the like. More specifically, turntable 314 is formed by a metallic receiving base connected to turntable motor 315 and a plate made of glass or the like placed on the metallic receiving base.
The microwaves are reflected by metal. In a microwave oven as the one shown in FIG. 14, the microwaves irradiated on a portion where a hole width of the metallic receiving base provided at the bottom of turntable 314 is not more than .lambda./2 (.lambda. is a wavelength of the microwave) are reflected by the receiving base so that the microwaves are not absorbed by object to be heated 316. In other words, in the microwave oven as the one shown in FIG. 14, there exist in the region on turntable 314 a region which absorbs from the lower side the microwaves irradiated from below turntable 314 and a region which does not absorb the microwaves.
As a result, the amount of the microwaves absorbed by the object to be heated greatly varies even with the same heating time, depending on the position at which the object to be heated is placed on turntable 314 in the microwave oven shown in FIG. 14. In short, the same problem of unstable heating capabilities occurs in the microwave oven shown in FIG. 14.